My understanding is, if you possess a prescription drug without a script then that is, at that point, relative to you, an illegal drug. Sure, no one is going to do time for, say, thyroid meds, but opoids is a whole other story.
This is why is cracks me up when people want to tie public services (e.g., food stamps) to drug testing. Imagine how many (white) middle class kids would be denied school is their parents' blood/piss tested positive for drugs they had no script for. Lol